How to shield my guitar with LEDs

Doodlegame

Hi,
Got my project guitar working over the weekend and it's sounding pretty good!  I'm having one issue with it though and that is that when you turn the USB powered LED lights on (the type that is a strip and has sticky back stuff) you get a ton of hum through the pups.  Is there any way to shield this or have I just got a normal guitar with a USB lead in it now..?  It seems stupid not to have thought of this now but ah well...

Cheers!

Rocker

Does the hum occur if you use a battery to power the LEDs?

Doodlegame

Rocker said:

Does the hum occur if you use a battery to power the LEDs?

I've not but I don't have an adapter, might try to buy one and see how that goes though....

WeZ84

The signal turning the LEDs on will might be a PWM (Pulse Width Modulated) signal.
So rather than a constant DC voltage, it is like a square wave - so the DC is fluctuating between on and off very quickly.
The longer the signal is in the On state, the brighter the LEDs will appear. That is a possible cause of the hum you are hearing.

If it is possible to adjust the brightness of the LEDs, try setting them to the maximum brightness - this will get the signal closer to a constant DC signal and may reduce the hum. 

jpfamps

I had a similar problem with a project controlling LEDs in an optocoupler with PWM where the PWM frequency was coupling into the audio.

I resolved it by increasing the PWM frequency out of the audio range.

I was worried that the LEDs would not turn on / off quick enough, but actually it was fine. 

There will be a loss of efficiency at higher PWM frequencies, which could a problem with stage lighting where significant power needs to be delivered, but not really an issue with a small number of LEDs.

In my experience USB sockets do not provide very clean power, and are very noisy, especially when drawing significant current. 

If the LEDs are not PWM controlled, you may be able to clean up the power using some form of RC or LC filtering.

Doodlegame

WeZ84 said:

The signal turning the LEDs on will might be a PWM (Pulse Width Modulated) signal.
So rather than a constant DC voltage, it is like a square wave - so the DC is fluctuating between on and off very quickly.
The longer the signal is in the On state, the brighter the LEDs will appear. That is a possible cause of the hum you are hearing.

If it is possible to adjust the brightness of the LEDs, try setting them to the maximum brightness - this will get the signal closer to a constant DC signal and may reduce the hum. 

jpfamps said:

I had a similar problem with a project controlling LEDs in an optocoupler with PWM where the PWM frequency was coupling into the audio.

I resolved it by increasing the PWM frequency out of the audio range.

I was worried that the LEDs would not turn on / off quick enough, but actually it was fine. 

There will be a loss of efficiency at higher PWM frequencies, which could a problem with stage lighting where significant power needs to be delivered, but not really an issue with a small number of LEDs.

In my experience USB sockets do not provide very clean power, and are very noisy, especially when drawing significant current. 

If the LEDs are not PWM controlled, you may be able to clean up the power using some form of RC or LC filtering.

Thanks guys, the brightness is adjustable so I'll try that, otherwise I'll have a mess round with RC LC filtering looking up alternative DC powered solutions..!

Thanks so much for the help

jpfamps

Doodlegame said:

WeZ84 said:

The signal turning the LEDs on will might be a PWM (Pulse Width Modulated) signal.
So rather than a constant DC voltage, it is like a square wave - so the DC is fluctuating between on and off very quickly.
The longer the signal is in the On state, the brighter the LEDs will appear. That is a possible cause of the hum you are hearing.

If it is possible to adjust the brightness of the LEDs, try setting them to the maximum brightness - this will get the signal closer to a constant DC signal and may reduce the hum. 

jpfamps said:

I had a similar problem with a project controlling LEDs in an optocoupler with PWM where the PWM frequency was coupling into the audio.

I resolved it by increasing the PWM frequency out of the audio range.

I was worried that the LEDs would not turn on / off quick enough, but actually it was fine. 

There will be a loss of efficiency at higher PWM frequencies, which could a problem with stage lighting where significant power needs to be delivered, but not really an issue with a small number of LEDs.

In my experience USB sockets do not provide very clean power, and are very noisy, especially when drawing significant current. 

If the LEDs are not PWM controlled, you may be able to clean up the power using some form of RC or LC filtering.

Thanks guys, the brightness is adjustable so I'll try that, otherwise I'll have a mess round with RC LC filtering looking up alternative DC powered solutions..!

Thanks so much for the help

If the brightness is adjustable then it's almost certainly PWM, which will make eliminating the noise by shielding hard unless you can increase the PWM frequency (and even then it could still cause problems).

It may be possible to use a regulated DC supply, but this will mean you would not have control over the brightness.